The present invention relates generally to a low pressure accumulator and more particularly, to a low pressure accumulator assembly for receiving and temporarily storing fluid in a vehicle anti-lock brake system.
Braking a vehicle in a controlled manner under adverse conditions such as rain, snow, or ice generally requires precise application of the brakes by the vehicle driver. Under these conditions, or in panic stop situations, a driver will often apply excessive brake pressure, thus causing the wheels to lock such that excessive slippage between the wheels and the road surface takes place. Wheel lockup conditions can lead to loss of directional stability and, possibly, uncontrolled vehicle spinout.
In a continuing effort to improve the operational safety of vehicles, many companies have been involved in the development of anti-lock braking systems. While typically such systems are adapted to control the braking of each braked wheel of a vehicle, some systems have been developed for controlling the braking of only a portion of the braked wheels. Examples of prior art anti-lock brake systems are disclosed in U.S. Pat. Nos. 3,515,440; 3,731,979; 3,870,376; and 3,880,474 and are herein incorporated by reference.
Generally, prior art anti-lock brake systems include a central control unit for monitoring the speed of the controlled wheels to determine the deceleration of the controlled wheels. When the brakes of the vehicle are applied and the wheel deceleration of the monitored wheels exceeds a predetermined deceleration threshold, indicating that there is wheel slippage and the wheels are approaching a lockup condition, the central control unit functions to control the application of hydraulic pressure through a control valve means to the associated brakes to prevent lockup of the controlled wheels. Typically, the anti-lock brake system includes means for cyclically reducing and reapplying pressure to the associated brakes to limit wheel slippage to a safe level while continuing to produce adequate brake torque to decelerate the vehicle as desired by the driver. While some systems utilize a separate hydraulic pump as the means for reapplying pressure, other systems, such as disclosed in U.S. Pat. No. 4,418,966, do not require the use of a separate hydraulic pump. U.S. Pat. No. 4,418,966 is herein incorporated by reference.
In controlling the application of pressure to selected wheel brakes, many systems utilize a low pressure accumulator which is operative to temporarily receive and store brake fluid during pressure reduction phases of the anti-lock operation. The low pressure accumulator typically maintains fluid stored therein at a predetermined minimum pressure determined by a compression spring acting on an accumulator piston, generally in the range of 30-60 p.s.i. This minimum pressure represents the lowest pressure to which the controlled brakes can be reduced during anti-lock operation. However, it has been found that, in certain braking situations, it is necessary to reduce the controlled pressure below this minimum pressure in order to achieve the desired control. U.S. Pat. No. 4,976,501, incorporated herein by reference, describes an accumulator which maintains brake fluid stored therein at or near zero pressure. Thus, if necessary, fluid pressure to the controlled wheel brakes can be reduced to near zero pressure.
Referring now to FIG. 1, a pair of conventional low pressure accumulators are generally shown at 100. Each low pressure accumulator 102 includes a generally cup-shaped piston 104 slidably received within an axial bore 106 of a hydraulic control unit (HCU) 108 of a braking system. An end cap 110 is sealingly mounted at an open end of the bore 106. Typically, the end cap 110 includes an aperture for receiving an elastomeric boot 112. The elastomeric boot 112 is generally cup shaped and has a closed end and an open end. The open end includes an inwardly extending flange for attaching to a lip portion (not shown) formed about the aperture of the end cap 110. The boot 112 may include a passageway, such as a narrow slit, for the passage of air in the closed end thereof. In operation the elastomeric boot 112 allows a small volume of air to flow between the boot 112 and the end cap 110 as the piston moves from a top dead center to a bottom dead center position. The flow of air is further facilitated in boots 112 having a narrow slit in the closed end. The very small opening between the boot 112 and the end cap 10 further inhibits, but does not eliminate, the flow of water or contaminants into the accumulator. The boot 112 thereby allows for the passage of air as the piston 104 moves and provides an environmental seal which prevents water and contaminants from entering the accumulator 102.
Conventional braking system hydraulic control units are typically mounted to vehicle frame by a plurality of bolts. Elastomeric grommets are typically mounted between each bolt and the frame to absorb vibrations. Such conventional braking system hydraulic control units typically require three or more mounting bolts and associated elastomeric grommets, washers, and nuts.
The invention relates to a fluid accumulator, especially a low pressure fluid accumulator of a hydraulic control unit of a vehicle brake system. The accumulator body has an inner surface defining an axial bore extending from an open end to a closed end of the accumulator body. A generally cup-shaped cylindrical piston has an inner surface defining an axial bore and extends from an open end to a closed end of the piston, and is slidably received within the axial bore of the accumulator body. A cap has a generally cylindrical body portion and an inner surface defining an axial passageway therethrough. A circumferential, radially outwardly extending flange is formed on one end of the body portion of the cap. The cap is mounted in the bore of the accumulator body with the outer edge of the flange of the cap pressed into or otherwise fixed relative to the bore of the accumulator body and retains the piston therein. A spring is disposed between the cap and the piston and urges the piston relatively away from the cap and toward the closed end of the accumulator body. A grommet has a shaft and an enlarged annular head formed on a first end of the shaft. The shaft is disposed to seal against the inner surface of the cap. The grommet has an inner surface defining an axial bore extending from the head through an open end of the shaft at the first end of the shaft to a closed end of said shaft at a second end of the shaft, the closed end of said shaft having a closable passageway therethrough.
The low pressure accumulator of the invention provides an advantageous integration of the mounting of a braking system hydraulic control unit, and venting of a low pressure accumulator into one component. Assembly of a hydraulic control unit within a vehicle is simplified because, for example, one or more fasteners, such as a bolt and an associated grommet, washer, and nut, is eliminated. Further, the mechanical features of the grommet create a xe2x80x9cdual-ratexe2x80x9d effect that is more compliant for low amplitude vibration.